It's not too soon to start asking.

The great promise of autonomous cars is not that we could each own one in our own driveway – the 21st century's version of owning your own Model T, or your own color TV, or your own bulky Macintosh – but that no one would need to own one at all.

That's because when cars can drive themselves, they can drive off when we're done with them. They can pick up other people instead of sitting parked outside. We'll request them on-demand. They'll pull up out front, take us right where we want to go, then do the same thing for a hundred other passengers, a hundred times over. They'll behave, in other words, like sophisticated ride-share services – or like personalized mass transit.

I've daydreamed about this possibility a number of times with transportation geeks, and invariably we always wind up in the same, more sober place: If the autonomous cars of the future will come to look an awful lot like transit, then what will become of the transit we know now?

This isn't an entirely silly question in 2014. We make billion-dollar investments in new transit infrastructure because we expect to use it for decades. Metropolitan planning organizations are in the very business of planning 30 and 40 years into the future. The Washington Area Metropolitan Transit Authority recently released its dream map of subway service in the city for the year 2040. By then, autonomous cars – in some form – will surely be commonplace.

The question of what they'll mean for transit was actually on the program this year at the Transportation Research Board annual meeting in Washington, where several thousand transportation officials and researchers met to talk about state-of-the-art asphalts, biker behavior, and the infrastructure of the future. In one packed session, I heard Jerome Lutin, a retired longtime New Jersey Transit planner, say something that sounded almost like blasphemy.

"We’re just wringing our hands, and we’re going to object to this," he warned the room. "But the transit industry needs to promote shared-use autonomous cars as a replacement for transit on many bus routes and for service to persons with disabilities."

Someone in the back of the room did object that many paratransit passengers need human assistance along the way that an autonomous vehicle alone couldn't give them. But Lutin's broader point is a fascinating one: If autonomous cars can one day better perform some of the functions of transit, shouldn't we let them? Shouldn't we take the opportunity to focus instead on whatever traditional transit does best in an autonomous-car world?

"If you can’t get more than 10 people on a bus, or five people on a bus, then why bother running it?" Lutin asked me after his session. "You’re wasting diesel fuel."

The implication in this raises (at least) two more questions: Exactly where (and when) will it make sense for people to use buses or rail instead of autonomous cars? And if autonomous cars come to supplement these services, should transit agencies get into the business of operating them? In my initial daydream – where shared self-driving cars are whisking us all about – it's unclear exactly who owns and manages them.

Lutin sounds skeptical that transit agencies will be able to move into this space. "They don't adapt well to change," he says. They're also governed by rigid mandates that limit what they can do. A mass transit agency can't overnight start operating something that looks like a taxi service. Public agencies also must contend with labor unions, and labor unions likely won't like the idea of replacing bus routes with autonomous cars.

There's also another consideration.

"There's an opportunity for autonomous taxi services to make money," Lutin says. "And nobody wants the government to compete with private industry and make money. We barely tolerate toll road authorities. If it looks like we can trade in our buses for a fleet of autonomous vehicles, and we can drop fares and at the same time we can make money, somebody in the private sector is going to want that."

And if public transit agencies exist in part to subsidize a service the private sector won't provide, what if that service no longer needs a subsidy?

"It no longer needs to be a governmental function."

That would leave us then with the more traditional forms that transit already takes: buses, subways, light rail, street cars. Lutin is certain that we'll still need transit, particularly in dense cities. An autonomous car, after all, takes up as much physical space as a car with a human at the wheel. We'll be able to fit more autonomous cars on a given roadway, because they'll be smart enough to drive practically bumper-to-bumper without colliding into each other. But there's still a finite capacity on the road. And in densely populated areas, buses and subway cars will still be able to carry more people.

"Theoretically, a highway [lane] can carry 2,200 vehicles per hour," Lutin says. "Even if you go to 4,400 or 6,600 vehicles per hour, there’s still that limit."

So we'll still need transit to get people into the Loop in Chicago, or across the Bay Bridge into San Francisco, or onto the island of Manhattan. These are the things that transit already does best, and that it will still do best in the age of the autonomous car. What's more, the same technology that will bring us autonomous cars will make traditional transit better, too. When buses have the same autonomous, communicating power that cars will have, they'll be able to drive safely within inches of each other, too. Picture a dedicated Bus Rapid Transit lane with moving buses queued up end-to-end.

In this world, cars may start to function like transit, but buses could come to work like trains. And they're a lot cheaper to deploy.

Top image of a Nissan LEAF electric car with an autonomous driving system: Yuya Shino/Reuters

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